In the area of application-specific analog integrated circuits a tight control of the parameters of the IC manufacturing process maybe required. In some cases the accuracy requirements may exceed the process specifications or process capability. One way to deal with these problems is to implement some form of flexibility in the circuitry. A well-known example is the voltage-controlled oscillator. Here, a variable capacitor is used to enable a small variation in the oscillation frequency due to a variation in the control voltage. Similar principles can be applied to all other analog components such as filters, impedance matching networks amplifiers and the like.
All of these implementations however must rely on active circuitry (e.g. an accurate voltage source connected to a variable capacitor) that in-turn may only have a limited accuracy, reproducibility and supply-rejection. Often an accurate component that is independent of active circuitry is preferable, which must be trimmed to achieve the required accuracy.
For example, this is common practice in fabrication of Bulk-acoustic-wave filters and high-accuracy resistor in calibration standards. The trimming of the individual circuits can be performed with various techniques such as laser trimming, ion milling, ultrasonic cutters, customized masks, etc. However, all of these techniques are expensive due to the fact that they require specialized equipment and a complex mix of fabrication steps (the physical trimming process) and verification steps (electrical measurements). Moreover, they require a physical access to the die which makes a trimming after packaging rather difficult.
WO 04034404 discloses the use of a single-gate poly floating arrangement for trimming an analog circuit.